Abstract

Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and X-ray diffraction measurements were performed on single crystals of L(+)-tartaric, D(–)-tartaric, and monohydrate racemic (MDL-) tartaric acid. The exact crystal structures of the three acids, including the positions of all hydrogen atoms, were determined at room temperature. It was pointed out that one of O–H–O hydrogen bonds in MDL-tartaric acid has an asymmetric double-minimum potential well along the coordinate of proton motion. The weight losses due to thermal decomposition of L- and D-tartaric acid were observed to occur at 443.0 and 443.2 K, respectively, and at 306.1 and 480.6 K for MDL-tartaric acid. The weight losses for L- and D-tartaric acid during decomposition were probably caused by the evolution of 3H₂O and 3CO gases. By considering proton transfer between two possible sites in the hydrogen bond, we concluded that the weight losses at 306.1 and 480.6 K for MDL-tartaric acid were caused by the evaporation of half the bound water molecules in the sample, and by the evaporation of the remaining water molecules and the evolution of 3H₂O and 3CO gases, respectively.

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